(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of encapsulating chips.
(2) Description of the Prior Art
In packaging semiconductor chips frequent use is made of methods to encapsulate the chips in packages that are aimed at further usage of the encapsulated chips. These packages have to meet typical requirements of high speed processing environments and are therefore heavily influenced by considerations of cost, usability, quality, ease and repeatability of manufacturing, throughput speed and others.
One of the more commonly materials that is used for this purpose is resin. Resins occur freely in a natural environment where industrially applied resins are synthetically prepared and can be created with many properties that apply for a given application. Synthetic resins (such as alkyd resins or phenolic resins) usually have high molecular weight and may have some of the properties of natural resins. Synthetic resins however are typically very different from natural resins. Synthetic resins may be thermoplastic or thermosetting, they can be made by polymerization or condensation, and they are used mostly as plastics or the essential ingredients of plastic, in varnishes or other coatings, in adhesives and in ion exchange.
In the semiconductor industry, resins are frequently molded into particular forms or shapes that are used to house or package semiconductor chips. These completed molds then serve as chip carrier and may contain parts within the mold that facilitate or enable this function such as a die pad (to position the chip onto), metal extensions (lead fingers) that serve to interconnect the packaged chip with its surrounding electrical environment and means (wire bonding) for connecting the chip to the metal extensions.
It is thereby also common practice to adapt plastic or resin chip carriers to a high speed semiconductor manufacturing environment, for the main reason that this is the predominant environment that is being used to produce high volumes of semiconductor chips at a competitive price. The chip carriers must thereby also be adaptable to a variety of chip sizes, again to make the chip carrier acceptable from a cost point of view. To adapt the chip carrier to a high-speed manufacturing environment, the design must be such that no parts of the carrier can interfere with the manufacturing process due to protruding parts of the carrier. This could cause deformation of the protruding part in addition to slowing down the manufacturing process due to the required intervention to remove the offending carrier.
One of the most important considerations in the use of plastic chip carriers is the consideration of producing the carriers such that they meet design considerations of surface smoothness while surface of sub-components that are part of the carriers must remain free of spurious depositions that may limit or inhibit their proper functioning. The mold cavity frequently consists of two sections, an upper section and a lower section. The lower section forms, after molding, the vehicle for mounting the chip and for supporting lead fingers. These supporting components are inserted in the lower mold prior to the formation of the pre-molded plastic chip carrier. In Prior Art processes for making plastic chip carriers, resin bleeding, also referred to as flash, tends to occur over the surface of lead fingers of pre-molded, open cavity plastic chip carriers. Whereas the surface of lead fingers, which are used for interconnecting the chip with the surrounding terminal, is liable to be covered with resin flash during the filling of the encapsulant, the makers or suppliers of pre-molded plastic chip carriers usually require that the resin bleeding is removed through chemical, electro, and/or mechanical de-flash processes and thereby expose the bond area in order to make a smooth and reliable interconnect. Where a thicker layer of flash occurs on the lead wires, these approaches often do not lead to success and parts must be scrapped. This in turn leads to significant yield loss and to a significant decrease in product throughput. The invention addresses these problems caused by resin bleeding.